1. Field of the Invention
The present disclosure relates to a sheet stacking apparatus having a function of aligning discharged sheets.
2. Description of Related Art
Conventionally, a sheet post-processing apparatus (finisher) is discussed. After receiving a sheet from an image forming apparatus, the finisher discharges the sheet onto a discharge tray and causes alignment members arranged on the discharge tray to align the sheet in a direction perpendicular to the discharging direction. Since the alignment members arranged on the discharge tray align such discharged sheets, the sheets can be stacked neatly on the discharge tray. Such alignment processing is referred to as “neat stack.”
An image forming apparatus uses heat to fix a toner image formed on a sheet onto the sheet. Thus, because of the heat added for fixing, a sheet transferred from the image forming apparatus to the finisher may be curled on a discharge tray.
FIGS. 14A and 14B illustrate states of sheets P stacked on a stacking tray 731. In FIG. 14A, the stack of sheets P on the discharge tray 731 is not curled. The discharge tray 731 is controlled to be positioned so that a sheet surface detection sensor 732 does not detect a sheet. Thus, the position of the top surface sheet of the stacked sheets P is maintained at approximately the height of the sheet surface detection sensor 732.
In contrast, in FIG. 14B, the stack of sheets P on the discharge tray 731 is curled. Based on a signal from the sheet surface detection sensor 732, the discharge tray 731 is lowered to a position where no sheet trailing edge blocks the discharge port. Thus, as illustrated in FIG. 14B, the position of the discharge tray 731 on which the curled sheets Pare stacked is lower than that of the discharge tray 731 on which the curled sheets P are not stacked. Therefore, depending on curling of the sheets P, even if an alignment member 730 moves in a width direction perpendicular to the sheet conveyance direction, the alignment member 730 does not come into contact with the side of the stacked sheets P. As a result, since the alignment member 730 executes a missed swing, an appropriate alignment operation may not be executed.
In response to this problem, U.S. Pat. No. 7,681,881 discusses arranging, other than the sheet surface detection sensor 732, a detection mechanism for detecting the position of the top surface of the stacked sheets at a position where the alignment member 730 comes into contact with the sheets in a sheet discharging direction. Based on the position of the sheet detected by the detection mechanism, the position of the alignment member 730 is adjusted vertically. Thus, if strongly curled sheets are stacked, the alignment member 703 is controlled to be lowered further, compared with when no curled sheets are stacked. Thus, when an alignment operation is executed, the alignment member 730 is prevented from executing a missed swing.
However, U.S. Pat. No. 7,681,881 requires a dedicated detection mechanism for detecting the position of the sheet surface at the position where the alignment member 730 comes into contact with the sheets, requiring a more complex configuration and increasing the cost.